Generally speaking, data patterns and indicia such as bar code patterns, data matrix patterns, OCR (Optical Character Recognition) fonts, text characters, graphic images, logos and other one dimensional (1D) and two dimensional (2D) patterns of geometric and graphic data, which may be referred to herein with the term “symbols,” are useful in a wide variety of applications. Some printers may be specialized for efficient printing of the data patterns.
Bar code printers may thus be widely deployed in various supply chain and identification applications. Printers for bar codes and other such symbols and indicia may use web printing techniques to apply markings related to writing, symbols, graphics, and other indicia onto a theretofore unmarked or pre-printed printable medium. The printable medium may thus comprise a web.
As used herein, the term “web” may refer to a strip of printable material that is significantly longer than it is wide. The web medium may be stored ready to use on a spool, magazine, fan-fold media, or other package or storage means. As printing is performed, the web medium is drawn from the storage means and fed longitudinally into the printer, where it is exposed to a printhead.
The printheads are operable for applying the markings controllably over at least one substantially planar surface of the printable web medium. The printheads may operate according to any mark application technology such as ink drop, matrix dot impact, toner, and any one of the various DPM (Direct Part Mark) printing technologies. The printheads of some bar code printers are operable thermally. The printable web medium used with such thermal printers comprises a heat sensitive material.
As the heat sensitive web medium is drawn into proximity with the thermal printhead, portions of its surface are marked controllably by selective heating therewith. The appearance of the surface portions may thus be darkened from a light shade (e.g., white or near-white) to print the markings. Alternately, the thermal printhead heats a thermally sensitive ribbon, which then transfers the heated image to a plain media substrate comprising the web. The printed web medium then emerges from the printer as a finished printing product.
Bar codes and other symbols may be printed and used as labels. The labels may comprise portions of the web medium, which may be separated therefrom as independent segments. A number of factors can affect the quality of the printing and thus the accuracy with which the markings are applied to the medium. Accurate printing allows labels to convey information correctly.
On the other hand, inaccurate printing may reduce the legibility of a label and cause errors in reading the information the label is intended to convey or provide graphics that are not aesthetically pleasing. The accuracy of the printing and the legibility of the label in relation to the information it is intended to present may be verified. For example, bar codes and OCR printed to particular specifications may be tested for compliance.
Verifying labels upon their emergence from the printers however adds cost, latency and complexity and occurs after the labels have already been printed, which can be wasteful. In-printer compliance testing techniques have thus been developed, with which verification of the labels is performed prior to their emergence from the printer.
In-printer compliance testing techniques may comprise capturing images of the nascent labels within the printer and during the print process. The captured images are evaluated for verification of the labels. In addition to capturing the verification images of each label, the in-printer verification may collect corresponding useful compliance testing statistics for every label printed.
In-printer compliance testing techniques may use the motion of the web medium to trigger the imaging of labels for verification and concomitant print quality correction in real-time. For example, the label may be imaged, digitized and sampled over at least part of the symbol, writing or graphic (hereinafter, “symbol”) presented by the label or other product.
The imaged symbol portion is compared to a print command and related update data thus generated, with which print logic may be updated in real time to control the quality of the printing and thus the accuracy of the label. The verification imaging and the real time quality adjustments are responsive to the motion of the web medium. However, the motion may not be consistently linear.
In fact, the motion of the web medium may tend to be non-linear in some respects. For example, thermal printers may slow down as the end of the label approaches and in some other situations. The verification images made under these circumstances may become distorted, the real time print quality adjustment unreliable and thus, the in-printer label verification may be inaccurate.
Therefore, a need exists for accurate compliance testing of symbols printed on media prior to the production of printed items, which emerge from the printer. A need also exists for the capturing verification images of symbols, which uses the motion of web media upon which the symbols are printed. Further, a need exists for considering non-linearity in the motion of the web media in the capture of the verification images of the printed symbols and compensating for such non-linear movement.